Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide. The probe is generally arranged to extend vertically from the top towards the bottom of the tank. The probe may also be arranged in a measurement tube, a so-called chamber, that is connected to the outer wall of the tank and is in fluid connection with the inside of the tank.
The transmitted electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and reception of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity of the electromagnetic signals.
Most radar level gauge systems on the market today are either so-called pulsed radar level gauge systems that determine the distance to the surface of the product contained in the tank based on the difference in time between transmission of a pulse and reception of its reflection at the surface of the product, or systems that determine the distance to the surface based on the phase difference between a transmitted frequency-modulated signal and its reflection at the surface. The latter type of systems are generally referred to as being of the FMCW (Frequency Modulated Continuous Wave) type.
In any case, the propagated electromagnetic signal is typically not only reflected at the impedance transition constituted by the interface between atmosphere and surface, but at several other impedance transitions encountered by the signal. In the case of a GWR-system, one such impedance transition typically occurs at the connection between the transceiver and the probe. Generally, the transceiver is located outside the tank, and is connected to the probe via a feed-through going through a wall (typically the roof) of the tank.
Such a feed-through is typically formed by a coaxial line having the probe as its inner conductor, the tank wall or a connection piece that is attached to the tank as its outer conductor, and a dielectric member provided between the inner and outer conductors.
Because of its structure, the impedance of the feed-through is similar to that of a typical coaxial cable, that is, about 50Ω.
Since the impedance of the probe is typically considerably higher (about 200-300Ω for a twin line probe and about 300-350Ω for a single line probe) there will be a relatively large impedance transition at the interface between the feed-through and the probe.
As explained above, this impedance transition partly reflects the transmitted electromagnetic signal, giving rise to an echo signal which may be substantially stronger than the surface echo signal resulting from reflection of the transmitted signal at the surface of the product contained in the tank, especially when the product to be gauged is a material that yields a relatively weak echo signal. Examples of products yielding relatively weak echo signals are liquid natural gas (LNG), liquid petroleum gas (LPG), oil-based products, solids such as plastic pellets or grain etc.
This may in turn make it difficult to determine the filling level when the surface of the product is close to the ceiling of the tank, and may, furthermore, reduce the maximum measurable distance (minimum measurable filling level) because of the loss of signal that occurs at the impedance transition between the feed-through and the probe.
For instance, for the situation described above, with an impedance transition from the 50Ω impedance of the feed-through to the 300Ω impedance of a single line probe, only about 25% of the transmitted power is available to return to the transceiver after having passed the impedance transition going out and coming back. If the impedance transition is reduced from 1:6 to 1:2, 80% of the power returns. The difference in maximum measurable distance between these cases can be as much as 10 meters.
Various impedance matching arrangements have been proposed to smooth the impedance transition at the connection between the transceiver and the probe in order to reduce the amplitude of the echo signal resulting from reflection of the transmitted signal at the impedance transition.
U.S. Pat. No. 6,681,626 discloses one such impedance matching arrangement for a GWR-type radar level gauge system with a single line probe, according to which at least one electrical conductor is arranged to be spaced apart from the single line probe inside the tank.
Moreover, U.S. Pat. No. 6,750,657 discloses other impedance matching arrangements for a GWR-type radar level gauge system with a single line probe, in which the feed-through is modified to provide a gradually increasing impedance from the transceiver and towards the inside of the tank. According to one embodiment, the probe diameter decreases through the feed-through, and according to another embodiment, the probe is positioned off-center in the feed-through and a second metallic guiding element which is tapered is positioned alongside the probe to form a two-wire line with increasing impedance through the feed-through.
A drawback of the impedance matching arrangements disclosed in U.S. Pat. No. 6,681,626 or U.S. Pat. No. 6,750,657 is, however, that they all require a relatively large through-hole and therefore are not suitable for tank installations in which a small through-hole (sometimes as small as having a diameter of 1″-2″ (2.5-5 cm) is used.